This invention relates to transcutaneous energy transfer (TET) devices and, more particularly, to such a device which includes a mechanism for protecting components mounted within a secondary coil from magnetic field induced heating.
Many medical devices are now designed to be implantable including pacemakers, defibrillators, circulatory assist devices, cardiac replacement devices such as artificial hearts, cochlear implants, neuromuscular simulators, biosensors, and the like. Since almost all of the active devices (i.e., those that perform work) and many of the passive devices (i.e., those that do not perform work) require a source of power, inductively coupled transcutaneous energy transfer (TET) and information transmission systems for such devices are coming into increasing use. These systems consist of an external primary coil and an implanted secondary coil separated by an intervening layer of tissue.
One problem encountered in such TET systems is that the best place to locate control circuitry for converting, amplifying and otherwise processing the signal received at the secondary coil before sending the signal on to the utilization equipment is within the secondary coil itself. However, there is also a significant magnetic field in the secondary coil resulting from the current induced therein, which field can induce heating of the components, particularly metallic components. As a minimum, such heating can influence the performance of various components, and in particular interfere with the desired uniform power applied to the equipment. In a worst case, the heating can be severe enough to cause damage or destruction to the components which can only be repaired or replaced through an invasive surgical procedure. Such heating can also cause injury or discomfort to the patient in which the components have been implanted.
Heretofore, in order to avoid such heating, it has either been necessary to be sure that the signal induced in the secondary coil is not sufficient to generate a magnetic field which would cause potentially damaging heating of the components or to mount the components at a less convenient location. The former is undesirable because it is generally not possible to eliminate significant heating of the components while still operating the device at required energy levels, and the later solution is not desirable since the output signal from the secondary coil can reach 500 volts and above at an operating frequency that can be in excess of 100 kHz. It is preferable that such high voltage signal not pass extensively through the body and it is difficult to provide good hermetically sealed connectors for signals at these voltages. It is therefore preferable that an auxiliary signal processing module, which may reduce the voltage to a value in the approximately 20 volt range, be included as close to the secondary coil as possible, a position inside the secondary coil being ideal for this purpose.
A need therefore exists for an improved technique for use with TET devices so as to enable at least selected electronic components to be mounted within the secondary coil with minimal heating of such devices.
In accordance with the above, this invention provides a transcutaneous energy transfer device which includes an external primary coil to which energy to be transferred is applied, an implanted secondary coil inductively coupled to the primary coil, each of the coils generating a magnetic field, and electronic components subcutaneously mounted within the secondary coil, with a mechanism being provided which reduces inductive heating of such components by the magnetic field of the secondary coil. For one embodiment of the invention, the mechanism for reducing inductive heating includes a cage formed of a high magnetic permeability material in which the electronic components are mounted. The material of such cage is preferably a ferromagnetic material such as a ferrite and is preferably sufficiently thick so that magnetic field values in the material are well below saturation and so that significant heat dissipation in the material does not occur. The material of the cage should however be as thin as possible while satisfying the above criteria. The cage may be thicker in areas of the cage experiencing high flux density and thinner in other areas. The cage may also be formed of a layer of ferromagnetic material laminated with at least one layer of a low magnetic conductivity material to enhance flux guidance.
Alternatively, the mechanism may include winding the secondary coil with a first number N1 of outer windings and a second number N2 of counter-wound inner windings, N1 being larger than N2. N1, N2 and the diameters of both the outer and inner windings are selected such that the magnetic field caused by the coils in the region of the components is reduced sufficiently to prevent significant component heating. For a preferred embodiment, N1, N2, and the diameters of the windings are selected so that the magnetic fields caused by the windings substantially cancel in the region of the components. For an illustrative embodiment, N1 is approximately 19, N2 is approximately 7, and the diameter of the outer winding is approximately 2.5 inches, and the diameter of inner winding is approximately 1.5 inches. Alternatively, the inner windings may be in the magnetic field of the outer winding, but not electrically connected thereto.
In another aspect of the invention, a transcutaneous energy transfer device is disclosed. The TET includes an external primary coil; an implantable secondary coil coupled to the primary coil; a cage formed of a high magnetic permeability material located within the secondary coil to reduce inductive heating of electronic components mounted therein caused by a magnetic field of the primary and secondary coils, wherein the cage has walls of varying thickness such that a lowest total mass is achieved without exceeding the saturation density of the cage material. In one embodiment, the thickness of the cage walls is a minimum thickness that results in magnetic flux density through the cage walls is approximately equal to the saturation density.
In another aspect of the invention, another transcutaneous energy transfer device is disclosed. This TET includes an external primary coil; an implantable secondary coil coupled to the primary coil; and a cage formed of a high magnetic permeability material located within the secondary coil to reduce inductive heating of electronic components mounted therein caused by a magnetic field of the primary and secondary coils, wherein the cage has a geometry configured to maximize permeability in flux pathway between the primary and secondary coils. In one embodiment, the cage has flanges that extend the high permeability shield material within the flux pathway.
In another embodiment of this aspect of the invention, the cage includes a cylindrical base; a lid shaped in the form of a disk; and the flanges integral with the base. The flanges extend a high magnetic permeable region from base into the magnetic flux pathway. Preferably, the flange is in-line with a shortest flux pathway between the primary and secondary coils, such as extending from base immediately adjacent to the secondary coil to guide the magnetic flux lines back toward the primary coil.
In another aspect of the invention, a transcutaneous energy transfer device is disclosed. This TET includes an external primary coil; an implantable secondary coil coupled to the primary coil; and a cage formed of a high magnetic permeability material within the secondary coil to house electronic components. The cage includes a base; and a self-aligning lid.
In one embodiment of this aspect of the invention, the base is cylindrical and the lid is shaped in the form of a disk. In anther embodiment, the base includes vertical walls. The lid includes an annular recessed shelf circumferentially formed around a mating surface of the lid configured to receive the vertical wall of the base.
In a stiff further aspect of the invention, a transcutaneous energy transfer device is disclosed. The device includes an external primary coil; an implantable housing formed of a substantially low thermal conductivity medium; a secondary coil, mounted within the implantable housing, coupled to the primary coil; a cage formed of a high magnetic permeability material within the secondary coil to house electronic components; and a heat distribution layer thermally coupled to the cage and to an internal surface of the housing. The heat distribution layer may be comprised of multiple alternating layers of high and low heat conductivity materials.
In a further aspect of the invention, a transcutaneous energy transfer system is disclosed. The TET includes a primary coil and an implantable secondary coil having an outer first winding having a first number of turns and a first diameter and an inner second winding having a second number of turns and a second diameter. A method for determining the second number of turns, includes the steps of: a) winding the first winding with a predetermined number of turns; b) inserting a magnetic field monitoring device in a central region of the secondary coil; c) applying a dc current through the first winding while monitoring a magnetic field in the central region; d) winding the second winding in a direction a direction of the first winding using a wire extension from the first winding; e) monitoring, as the second winding is wound in the step d), a strength of a magnetic field in the central region; and f) stopping the winding of the second winding when the magnetic field strength reaches approximately zero.
Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the drawings, like reference numerals indicate like or functionally similar elements. Additionally, the left-most one or two digits of a reference numeral identifies the drawing in which the reference numeral first appears.